fix(frontend)!: Restrict numeric generic types to unsigned ints up to…

… `u32` (#5581)

# Description

## Problem\*

Resolves #5571 

The issue references expanding the supported type-level integers,
however, after a team discussion we decided to simply restrict the
supported numeric generics. If other kinds of numeric generics become
highly requested we can then re-assess whether we want to expand support
for type level constants.

## Summary\*

A new parser combinator for numeric generics was introduced to
differentiate with the existing `type_expression` combinator. This is
due to usage of `or_not` in the `generic_type_args` parser. Due to the
`or_not` combinator the expression error was being ignored. Thus, the
new numeric generic expression combinator returns an optional expression
along with its parsed type.

## Additional Context

This program:
```rust
    fn big<let N: u64>() -> u64 {
        N
    }

    fn main() {
        let _ = big::<18446744073709551615>();
    }
```
gives the following error:
<img width="972" alt="Screenshot 2024-07-23 at 12 18 00 PM"
src="https://github.com/user-attachments/assets/fbbaf78a-ab86-4250-9008-ba4d2751409d">

The previous errors were very unclear:
<img width="725" alt="Screenshot 2024-07-22 at 5 59 04 PM"
src="https://github.com/user-attachments/assets/c532de18-0240-4a4a-acbc-f0be72b5b2db">



## Documentation\*

Check one:
- [ ] No documentation needed.
- [ ] Documentation included in this PR.
- [X] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [X] I have tested the changes locally.
- [X] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

---------

Co-authored-by: jfecher <[email protected]>

compiler/noirc_frontend/src/ast/mod.rs

@@ -301,15 +301,12 @@ impl UnresolvedTypeExpression {
     // This large error size is justified because it improves parsing speeds by around 40% in
     // release mode. See `ParserError` definition for further explanation.
     #[allow(clippy::result_large_err)]
-    pub fn from_expr(
+    pub(crate) fn from_expr(
         expr: Expression,
         span: Span,
     ) -> Result<UnresolvedTypeExpression, ParserError> {
         Self::from_expr_helper(expr).map_err(|err_expr| {
-            ParserError::with_reason(
-                ParserErrorReason::InvalidArrayLengthExpression(err_expr),
-                span,
-            )
+            ParserError::with_reason(ParserErrorReason::InvalidTypeExpression(err_expr), span)
         })
     }
 
@@ -323,7 +320,6 @@ impl UnresolvedTypeExpression {
 
     fn from_expr_helper(expr: Expression) -> Result<UnresolvedTypeExpression, Expression> {
         match expr.kind {
-            // TODO(https://github.com/noir-lang/noir/issues/5571): The `sign` bool from `Literal::Integer` should be used to construct the constant type expression.
             ExpressionKind::Literal(Literal::Integer(int, _)) => match int.try_to_u32() {
                 Some(int) => Ok(UnresolvedTypeExpression::Constant(int, expr.span)),
                 None => Err(expr),

compiler/noirc_frontend/src/parser/errors.rs

@@ -20,8 +20,8 @@ pub enum ParserErrorReason {
     MissingSeparatingSemi,
     #[error("constrain keyword is deprecated")]
     ConstrainDeprecated,
-    #[error("Expression is invalid in an array-length type: '{0}'. Only unsigned integer constants, globals, generics, +, -, *, /, and % may be used in this context.")]
-    InvalidArrayLengthExpression(Expression),
+    #[error("Invalid type expression: '{0}'. Only unsigned integer constants up to `u32`, globals, generics, +, -, *, /, and % may be used in this context.")]
+    InvalidTypeExpression(Expression),
     #[error("Early 'return' is unsupported")]
     EarlyReturn,
     #[error("Patterns aren't allowed in a trait's function declarations")]
@@ -44,10 +44,8 @@ pub enum ParserErrorReason {
     InvalidBitSize(u32),
     #[error("{0}")]
     Lexer(LexerErrorKind),
-    // TODO(https://github.com/noir-lang/noir/issues/5571): This error can be removed once support is expanded for type-level integers.
-    // This error reason was added to prevent the panic outline in this issue: https://github.com/noir-lang/noir/issues/5552.
-    #[error("Only unsigned integers allowed for numeric generics")]
-    SignedNumericGeneric,
+    #[error("The only supported numeric generic types are `u1`, `u8`, `u16`, and `u32`")]
+    ForbiddenNumericGenericType,
 }
 
 /// Represents a parsing error, or a parsing error in the making.

compiler/noirc_frontend/src/parser/parser/function.rs

@@ -4,8 +4,8 @@ use super::{
     parameter_name_recovery, parameter_recovery, parenthesized, parse_type, pattern,
     self_parameter, where_clause, NoirParser,
 };
-use crate::parser::spanned;
 use crate::token::{Keyword, Token};
+use crate::{ast::IntegerBitSize, parser::spanned};
 use crate::{
     ast::{
         FunctionDefinition, FunctionReturnType, ItemVisibility, NoirFunction, Param, Visibility,
@@ -91,10 +91,12 @@ pub(super) fn numeric_generic() -> impl NoirParser<UnresolvedGeneric> {
         .map(|(ident, typ)| UnresolvedGeneric::Numeric { ident, typ })
         .validate(|generic, span, emit| {
             if let UnresolvedGeneric::Numeric { typ, .. } = &generic {
-                if let UnresolvedTypeData::Integer(signedness, _) = typ.typ {
-                    if matches!(signedness, Signedness::Signed) {
+                if let UnresolvedTypeData::Integer(signedness, bit_size) = typ.typ {
+                    if matches!(signedness, Signedness::Signed)
+                        || matches!(bit_size, IntegerBitSize::SixtyFour)
+                    {
                         emit(ParserError::with_reason(
-                            ParserErrorReason::SignedNumericGeneric,
+                            ParserErrorReason::ForbiddenNumericGenericType,
                             span,
                         ));
                     }
@@ -228,7 +230,7 @@ mod test {
                 // fn func_name(x: impl Eq) {} with error Expected an end of input but found end of input
                 // "fn func_name(x: impl Eq) {}",
                 "fn func_name<T>(x: impl Eq, y : T) where T: SomeTrait + Eq {}",
-                "fn func_name<let N: u64>(x: [Field; N]) {}",
+                "fn func_name<let N: u32>(x: [Field; N]) {}",
             ],
         );
 
@@ -249,8 +251,12 @@ mod test {
                 "fn func_name<let T>(y: T) {}",
                 "fn func_name<let T:>(y: T) {}",
                 "fn func_name<T:>(y: T) {}",
-                "fn func_name<T: u64>(y: T) {}",
+                // Test failure of missing `let`
+                "fn func_name<T: u32>(y: T) {}",
+                // Test that signed numeric generics are banned
                 "fn func_name<let N: i8>() {}",
+                // Test that `u64` is banned
+                "fn func_name<let N: u64>(x: [Field; N]) {}",
             ],
         );
     }

compiler/noirc_frontend/src/parser/parser/types.rs

@@ -3,7 +3,9 @@ use super::{
     expression_with_precedence, keyword, nothing, parenthesized, path, NoirParser, ParserError,
     ParserErrorReason, Precedence,
 };
-use crate::ast::{Recoverable, UnresolvedType, UnresolvedTypeData, UnresolvedTypeExpression};
+use crate::ast::{
+    Expression, Recoverable, UnresolvedType, UnresolvedTypeData, UnresolvedTypeExpression,
+};
 use crate::QuotedType;
 
 use crate::parser::labels::ParsingRuleLabel;
@@ -201,8 +203,7 @@ pub(super) fn generic_type_args<'a>(
         // separator afterward. Failing early here ensures we try the `type_expression`
         // parser afterward.
         .then_ignore(one_of([Token::Comma, Token::Greater]).rewind())
-        .or(type_expression()
-            .map_with_span(|expr, span| UnresolvedTypeData::Expression(expr).with_span(span)))
+        .or(type_expression_validated())
         .separated_by(just(Token::Comma))
         .allow_trailing()
         .at_least(1)
@@ -234,7 +235,26 @@ pub(super) fn slice_type(
         })
 }
 
-pub(super) fn type_expression() -> impl NoirParser<UnresolvedTypeExpression> {
+fn type_expression() -> impl NoirParser<UnresolvedTypeExpression> {
+    type_expression_inner().try_map(UnresolvedTypeExpression::from_expr)
+}
+
+/// This parser is the same as `type_expression()`, however, it continues parsing and
+/// emits a parser error in the case of an invalid type expression rather than halting the parser.
+fn type_expression_validated() -> impl NoirParser<UnresolvedType> {
+    type_expression_inner().validate(|expr, span, emit| {
+        let type_expr = UnresolvedTypeExpression::from_expr(expr, span);
+        match type_expr {
+            Ok(type_expression) => UnresolvedTypeData::Expression(type_expression).with_span(span),
+            Err(parser_error) => {
+                emit(parser_error);
+                UnresolvedType::error(span)
+            }
+        }
+    })
+}
+
+fn type_expression_inner() -> impl NoirParser<Expression> {
     recursive(|expr| {
         expression_with_precedence(
             Precedence::lowest_type_precedence(),
@@ -246,7 +266,6 @@ pub(super) fn type_expression() -> impl NoirParser<UnresolvedTypeExpression> {
         )
     })
     .labelled(ParsingRuleLabel::TypeExpression)
-    .try_map(UnresolvedTypeExpression::from_expr)
 }
 
 pub(super) fn tuple_type<T>(type_parser: T) -> impl NoirParser<UnresolvedType>

compiler/noirc_frontend/src/tests.rs

@@ -1634,7 +1634,7 @@ fn numeric_generic_in_function_signature() {
 #[test]
 fn numeric_generic_as_struct_field_type() {
     let src = r#"
-    struct Foo<let N: u64> {
+    struct Foo<let N: u32> {
         a: Field,
         b: N,
     }
@@ -1695,7 +1695,7 @@ fn numeric_generic_as_param_type() {
 #[test]
 fn numeric_generic_used_in_nested_type_fail() {
     let src = r#"
-    struct Foo<let N: u64> {
+    struct Foo<let N: u32> {
         a: Field,
         b: Bar<N>,
     }
@@ -1745,7 +1745,7 @@ fn numeric_generic_used_in_nested_type_pass() {
 
 #[test]
 fn numeric_generic_used_in_trait() {
-    // We want to make sure that `N` in `impl<let N: u64, T> Deserialize<N, T>` does
+    // We want to make sure that `N` in `impl<let N: u32, T> Deserialize<N, T>` does
     // not trigger `expected type, found numeric generic parameter N` as the trait
     // does in fact expect a numeric generic.
     let src = r#"
@@ -1756,7 +1756,7 @@ fn numeric_generic_used_in_trait() {
         d: T,
     }
     
-    impl<let N: u64, T> Deserialize<N, T> for MyType<T> {
+    impl<let N: u32, T> Deserialize<N, T> for MyType<T> {
         fn deserialize(fields: [Field; N], other: T) -> Self {
             MyType { a: fields[0], b: fields[1], c: fields[2], d: other }
         }
@@ -2224,7 +2224,6 @@ fn impl_stricter_than_trait_different_trait_generics() {
         ..
     }) = &errors[0].0
     {
-        dbg!(constraint_name.as_str());
         assert!(matches!(constraint_typ.to_string().as_str(), "A"));
         assert!(matches!(constraint_name.as_str(), "T2"));
         assert!(matches!(constraint_generics[0].to_string().as_str(), "B"));